1. Field of the Invention
This invention relates to an improved RF (radio frequency) circuit that can be used in various transmitters. In particular, the invention relates to a method and apparatus for frequency stabilizing a Voltage Controlled SAW oscillator (VCSO) so that it can generate a stable RF carrier signal.
2. Description of the Prior or Contemporary Art
It has been recognized in the art that Voltage Controlled SAW Oscillators (VCSO) have an unstable output frequency, caused by age and thermal effects, which make them inappropriate for use as RF carrier oscillators. The following patents teach prior art techniques for correcting such frequency instability:
U.S. Pat. No. 4,011,526, issued to Kinsman, compensates for the parabolic temperature frequency drift characteristic of a surface acoustic wave (SAW) oscillator. The Kinsman apparatus includes a pair of varactor diodes and a temperature sensitive voltage supply. The diodes are connected in series and coupled to the SAW oscillator. The varactor diode's capacitance versus temperature characteristic provides a parabolic output signal. The parabolic output signal is used to compensate for the thermal drift of the SAW resonator.
U.S. Pat. No. 4,338,575, issued to Hartman, teaches a technique to compensate for temperature variations by coupling SAW resonators. A transmission line delay that varies with temperature is coupled to one of the SAW oscillators.
U.S. Pat. No. 4,193,045, issued to Houkawa et al, teaches the use of a parallel arrangement of SAW oscillators. Each SAW oscillator has a different oscillation frequency and a different turnover temperature so that a small frequency deviation occurs with temperature changes.
U.S. Pat. No. 4,489,289, issued to Slobodnick et al, uses a look-up table stored in an EPROM to provide frequency corrections to the SAW oscillator circuit. A temperature sensitive circuit decides what values to choose.
U.S. Pat. No. 4,639,697, issued to Yarranton et al teaches temperature compensation of a SAW oscillator by using passive and electronic phase shifters. The electronic phase shifter provides a phase shift as a function of temperature to compensate for instability of the SAW oscillator.